The present application relates to a process for increasing the firing shrinkage of ceramic slips for producing films by adding a high molecular weight mixed ester or a mixture of various such mixed esters to said slip.
Films composed of ceramic mixtures, for example of lead titanate, silicon carbide or aluminum oxide, can be produced by casting from so-called slips (slip casting). In this connection, slip is understood to mean a slurry of ceramic materials in a solvent (generally organic) which also additionally contains binders and plasticizers. Said slip is then poured, for example, onto a continuously travelling metal or plastic strip. On the strip, the slip is dried and a flexible ceramic film is produced which is continuously peeled off the strip. Such a ceramic film can readily be further processed with suitably shaped tools. The organic constituents are subsequently evaporated from the film by heating and the latter is finally sintered at high temperature to form the actual substrate. Film casting is dealt with in a general form in Keramischen Zeitschrift 38. No. 2 (1986), pages 79-82. Mixed phthalic acid esters are also mentioned therein as a plasticizing constituent of film casting mixtures.
The ceramic film contracts both during the evaporation of the organic constituents and also during sintering (shrinkage, in particular firing shrinkage). This means, however, that the firing shrinkage has to be allowed for in advance in producing the tools for shaping the films; i.e. that, for example, in producing a "ceramic card", the latter has to be punched out larger than the required "final dimensions" demand. In most cases, the "longitudinal shrinkage" and "transverse shrinkage" of a ceramic film is determined once and the tools are then designed accordingly, in other words, the firing shrinkage is regarded as a characteristic parameter of the ceramic film.
If the composition of the slip is now changed, be it by replacing one solvent by another or by replacing or varying the amount of the plasticizer or the binder or be it by using ceramic basic materials of different origin, different particle size or the like, the firing shrinkage will also alter and this has the result that the tools are no longer correctly dimensioned and the required "final dimensions" of the substrates are no longer maintained.
The production of new, correctly dimensioned tools is very cost-intensive and generally out of the question.
A reduction in the firing shrinkage may possibly still be achieved with relatively little difficulty by reducing, for example, the proportion of solvents.
An increase in the firing shrinkage, on the other hand, cannot usually be achieved in a simple manner--for example, by increasing the proportion of binder, plasticizer or solvent.